1. Field of the Invention
The present invention relates to a process for the synthesis of aromatic urethanes starting from an organic carbonate and an aromatic amine having formula (I-III), which allows high yields and selectivities to the useful reaction product.
2. Description of the Background
Aromatic urethanes (or carbamates) are valuable intermediates which can be used for the production of phytodrugs, dyes, pharmaceutical compounds and aromatic isocyanates used in the synthesis of polyurethanes.
Among aromatic urethanes, those of greatest interest, from a commercial point of view, are 4,4'-methylenediphenyldiurethane (MDA) and 2,4-toluenediurethane (TDA) used for the preparation of methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), which are at present industrially produced by phosgenation of the corresponding diamines.
Processes are known for the production of urethanes which are based on the functionalization of amines with a carbonate, preferably dimethylcarbonate (DMC), in the presence of suitable catalysts according to the following scheme: ##STR1##
For example, U.S. Pat. No. 3,763,217 discloses the preparation, under reflux conditions, of carbamates by the reaction of an alkyl carbonate with an amine, in the presence of an acid catalyst, preferably uranyl nitrate. Under these conditions however, the conversion and selectivity yields of the carbamate are about 20%.
According to U.S. Pat. No. 4,268,683, mono- and dicarbamates are prepared by the reaction of an alkyl carbonate and an aromatic mono- or diamine, using a compound of Sn (II) or Zn (II) as Lewis acid, such as for example halides or salts of monovalent organic acids with pKa equal to or higher than 2.8, preferably zinc acetate. Operating according to this process conversion and selectivity yields of about 77% are obtained for mono-carbamate and 18-36% for dicarbamate.
U.S. Pat. No. 5,091,556 discloses a synthesis method which comprises (i) a first step in which the carbonate is reacted with an aromatic, aliphatic or cycloaliphatic mono-amine, in the presence of zinc acetate, removing the alcohol produced during the reaction, to produce a mixture containing carbamate and urea, and (ii) a second step in which the mixture coming from the first step is reintegrated with the carbonate and reacted for a further 3-6 hours at 160.degree. C. to transform the urea into carbamate.
This system allows the production of aromatic mono-carbamates with good yields and selectivities. When a diamine or aromatic polyamine is used as substrate however, the selectivity into the corresponding urethane is reduced as large quantities of ureas or polyureas are formed which cannot be quantitatively and selectively retransformed into the corresponding urethane.
In accordance with the teachings of the known art discussed above, the reaction between carbonates and amines to produce the corresponding carbamates is typically carried out by the complete removal of the reaction alcohol (U.S. Pat. No. 3,763,217 and U.S. Pat. No. 5,091,556). In fact, the accumulation of alcohol in the reaction environment causes a lowering of the synthesis kinetics and a consequent decrease in the yields to urethane owing to the secondary reactions which can become competitive under these conditions.
The object of the present invention is to obtain aromatic urethanes with high yields and selectivities, by means of a simple, economic process which can be easily effected on an industrial scale and which does not have the disadvantages of the processes of the known art.
It has now been found that this object can be reached if the reaction between the organic carbonate and the aromatic amine is carried out by the partial removal of the alcohol coproduced in the reaction.
In accordance with this, the present invention relates to a process for the synthesis of aromatic urethanes which comprises:
(a) reacting an organic carbonate, in a stoichiometric quantity or higher than the stoichiometric value, with an aromatic amine having formula (I-III): ##STR2## wherein: R is hydrogen, a halogen, or a hydrocarbyl or hydrocarbyloxy group with up to 8 carbon atoms, preferably up to 4, A is a divalent hydrocarbon group with from 1 to 6 carbon atoms, preferably from 1 to 4, n has a value of 0 or 1 and x has a value between 1 and 6, in the presence of a Lewis acid catalyst, at a temperature of 100 to 190.degree. C.; PA1 (b) recovering the urethane from the reaction mixture; wherein said process is characterized in that the reaction in step (a) is carried out maintaining the reaction alcohol within values ranging from 10 to 40% in moles with respect to the total quantity of alcohol coproduced during the reaction.
Non-limiting examples of aromatic amines having formula (I-III) are: 4,4'-methylenedianiline, 2,4'-methylenedianiline, 2,4-diaminotoluene (TDA), 2,6-diaminotoluene or mixtures of the two isomers, 1,3-diaminobenzene, 2,6-diaminonaphthalene or a polymeric methylenedianiline (mixture of isomers having formula III with x=1, 2, 3 and 4).
Organic carbonates which can be used in the process of the present invention comprise alkyl, aryl or alkyl aryl esters of carbonic acid. The ester group can be an alkyl group with up to 12 carbon atoms, preferably up to 6, or an aryl group with up to 10 carbon atoms.
Examples of organic carbonates particularly suitable for the process of the present invention are cyclic or acyclic carbonates such as for example ethylene carbonate, propylene carbonate, styrene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, dibutyl carbonate, diisopropyl carbonate, dihexyl carbonate, methyl butylcarbonate, diphenylcarbonate and methylphenylcarbonate. The organic carbonates can be prepared using the known methods.
In the process of the present invention a quantity of carbonate in excess with respect to the amine is preferably used as it also acts as solvent.
Catalysts suitable for the purposes of the present invention generally consist of Lewis acids such as bivalent tin and zinc chlorides, or salts of these metals with mono- or bi-carboxylic organic acids. Among bivalent compounds of zinc, anhydrous or dihydrate zinc acetate is preferably used.
The quantity of catalyst can vary from 20 to 0.5% in moles and preferably from 10 to 1.0% in moles per mole of amine (I-III).
The reaction temperature can vary from 100.degree. to 190.degree. C. Temperatures of about 140-180.degree. C. are preferably used.
The process of the present invention, after initial pressurization with nitrogen at 1-3 absolute atmospheres (ata), is carried out at an operating pressure which is the autogenous pressure of the system and which depends on the substrate and operating conditions.
The reaction time depends on the temperature and pressure. However, reaction times of between 1 and 3 hours are sufficient.
The process of the present invention can be carried out in batch, in continuous or semicontinuous. At the end of the reaction, the product is recovered with the usual separation techniques. For example, the carbamate can be isolated by crystallization or evaporation of the solvent after elimination of the catalytic system.
The carbamoylation reaction can be carried out in a stainless steel autoclave, equipped with a distillation column arranged in series with a condenser.
In practice, the carbonate and amine can be premixed and the catalyst added to the mixture thus obtained. The whole mixture is charged into the reactor which, after initial pressurization, is brought to the operating temperature.
During the reaction a partial distillation of the alcohol is effected, in the form of an azeotropic mixture with the carbonate, so that the quantity of alcohol coproduced remaining in the reaction mixture is maintained at 10 to 40% in moles with respect to the total quantity of alcohol produced, preferably between 20 and 35% in moles.
Operating in this way, high yields and selectivities of the useful reaction product can be obtained, in a single step, in short times and with a production of urea of less than 1% by weight.
In addition, the carbonate which is removed from the reaction environment is not restored and consequently there is a concentration of the solution at the end equal to about 10%. This concentration has no negative effect on the process yield.
Finally the urethane contained in the reaction mixture is isolated and purified. The removal and consequent re-use of the carbonate is carried out by evaporation of the reaction mixture.